Alternative lixiviants to cyanide for leaching gold ores

Abstract

Over 25 alternative lixiviant processes to cyanide have been tested in the laboratory; some of which have been successful for niche applications. The process conditions, applications and current status of the most attractive are reviewed in this chapter with an emphasis on publications since 1995. Most work has focused on thiosulfate, thiourea, and halide processes. Oxidative chloride-, sulfide-, and ammonia-leaching processes are generally more applicable for the extraction of gold or platinum group metals as a byproduct from base metal sulfide concentrates. Despite the research interest and pilot plant trials on many of the noncyanide gold lixiviants, the majority are still at the development stage. The most advanced alternative lixiviant is thiosulfate leaching of carbonaceous preg-robbing ores, which has been largely developed by Newmont Mining, Placer Dome, and Barrick Gold. More recent development of a thiosulfate-leaching plant at Barrick Gold's Goldstrike operation has alluded to the complex and unstable nature of this method of leaching gold and the significant process design difference of using thiosulfate over conventional cyanide. Further work by the industry and its stakeholders is required to continue developing robust overall process flow sheets incorporating gold recovery, reagent recycling, and impurity control for a wider range of ore applications using alternative lixiviants to cyanide. Some chemicals used such as ammonia or elements leached from ore, such as mercury, also pose health, safety, and environmental concerns. Consequently, proper disposal of wastes and sustainable development issues also have to be considered in process designs.